In general, a solenoid converts electric energy into magnetic flux, release of which is transferred into linear mechanical motion of a plunger installed in the center of a C-frame solenoid, a D-frame solenoid, or a tubular solenoid (as shown respectively in FIG. 1A, FIGS. 1B, and 1C). Current flow I through the solenoid coil winding with inductance L creates magnetic energy       E    =                  1        2            ⁢              LI        2              ,which produces an attraction force Fmag between a movable plunger and a fixed stop. Solenoids typically have a working, or variable, air gap between the plunger and the stop, as well as a fixed air gap between the outside diameter of the plunger and either its frame or mounting bushing. To complete the magnetic circuit, the magnetic flux lines flow through either air or the metallic frame through the stop, the plunger, the frame or the mounting busing of a tubular solenoid and return to their point of origination.
The performance of a solenoid is dependent on numerous parameters, including, but not necessarily limited to, its physical size, the wattage applied, duty cycle, ambient temperature, its coil temperature due to heat rise, the coil ampere-turns (NI where I and N are current and coil turns respectively), solenoid orientation, cross sectional area of the plunger, the coil winding and the plunger and stop geometry. FIG. 2 illustrates typical force-stroke relationships for different geometries of plunger and mating stops of a D.C. solenoid.
Typically, the greater the holding force of a given plunger and stop geometry, the lesser the pulling/pushing force at an extended stroke position. In this regard, the minimum pull/push force generated is typically at the extended stroke end where the plunger assembly begins it's lifting towards the stop. As the plunger approaches the stop position, the pulling/pushing force developed typically increases dramatically, and the slope of the force-stroke curve rises sharply. The differential equations for an electrical circuit and Maxwell's equations for dynamics, which define the forces according to the current and position, describe the full dynamic or switching response of an electromechanical actuator. In fact, there is a certain transient time needed to develop magnetic flux and transfer it's energy to mechanical momentum.
In many applications this intrinsic transient phenomenon may ultimately effects the dynamics of other mechanical parts dependent on the plunger position and it's speed. One of these applications is related to high-pressure fuel injectors used in direct injection gasoline and diesel engines. In internal combustion engines (especially diesel engines) the transient phases, including injection, ignition (or auto-ignition) and combustion, have ultra-short time fractions from a few tens to a few hundreds of a nanosecond. In this regard, FIG. 3 shows data regarding normal heptane reactions starting at 900K and 83 bar in connection with a two stage CI (diesel) combustion process. More particularly, FIG. 3 relates to: (a) a first stage including premixed flame (0.03 ms) having various short-lived species such as C7 radicals, aldehydes (PAH), and hydrogen peroxide; and (b) a second stage including rapid oxidation (0.06 ms) having hydrogen, water, carbon dioxide, carbon monoxide, methane, soot precursors, C3-compounds, and C4-compounds.
Further, FIG. 4 depicts certain ideally targeted or aimed or purposed injection events (e.g., hampered by unstably controlled injection shot duration and dwell interval) and FIG. 5 depicts a diesel diffusion flame in connection with a conventional single long shot per cylinder injection (with limited access of air resulting in incomplete combustion).
Further still, one conventional electronically controlled diesel fuel injector is called an “accumulator” type. In these injectors, a nozzle includes an accumulator chamber that is charged with fuel under high pressure, which communicates with a nozzle port. An actuating device is associated with the injection valve and is moveable within a control chamber that is also pressurized with fuel. A valve is associated with the control chamber and is opened so as to reduce the pressure and cause the pressure in the accumulation chamber to unseat the injection valve and initiate fuel injection. Typically, a main electromagnetic assembly that is contained within the housing of the fuel injection nozzle operates the valve.
FIGS. 6A-6D depict four strokes of unit injector (“UI”) and unit pump (“UP”) operation stages. The function of these single-cylinder injection-pump systems can be subdivided into four operation stages (corresponding, respectively, to each of FIGS. 6A-6D):                a) Suction stroke. The follower spring (3) forces the pump plunger (2) upwards. The fuel in the fuel supply's low-pressure stage is permanently under pressure and flows from the low-pressure stage into the solenoid-valve chamber (6) via the bores in the engine block and the inlet (or feed) passage (7).        b) Initial stroke. The actuating cam (1) continues to rotate and forces the pump plunger (2) downwards. The solenoid valve is open so that the pump plunger (2) can force the fuel through the fuel-return passage (8) into the fuel supply's low-pressure stage.        c) Delivery and injection stroke (or Prestroke). An electronically timed signal from the engine electronic control unit (“ECU”) energizes the solenoid-valve coil (9) to pull the solenoid valve needle (5) towards the solenoid valve seat/stop (10). The connection between the high-pressure chamber (4) and the low-pressure stage is closed. Further movement of the pump plunger (2) causes increased fuel pressure in the high-pressure chamber (4); the fuel is also pressurized in the nozzle-needle (or nozzle assembly)(11). Upon reaching the nozzle needle opening pressure (typically over 300 bar), the nozzle needle (11) is lifted from its seat and fuel is injected into the engine combustion chamber. Due to the pump plunger's high delivery rate, the pressure continues to increase throughout the whole of the injection process (typically up to maximum peak of 1800-2000 bar).        d) Residual stroke. As soon as the solenoid-valve coil (9) is switched off, the solenoid valve (or solenoid valve needle) (5) opens after a short delay and opens the connection between the high-pressure chamber and the low-pressure stage.        
FIGS. 7A-7D relate to the above-mentioned operating stages of FIGS. 6A-6D and show, respectively, coil current (IS), solenoid-valve needle stroke (hM), injection pressure (Pe), and nozzle-needle stroke (hN).
FIG. 8 depicts a wave form diagram associated with operation of a fuel injector nozzle (an “accumulator” type injector) under use of two actuating solenoids installed into injector.
Finally, a number of conventional techniques and apparatuses achieve multiple injection, for instance, by using a piezoelectric actuator during individual injection phases or a rapid switching on/off of injection events strategy via the electronic control unit. Specifically with reference to application of rapidly operating electromagnetic actuators, studies have been carried out on variable valve actuators for valve train parts, rather than for high-pressure fuel injectors. Related documents include: 1) Robert Bosch GmbH (1999). Diesel-engine management. SAE, 2nd edition, 306 p.; 2) B. Riccardo, C.R.F. Societa' Consottile per Azioni (2000). Method of controlling combustion of a direct-injection diesel engine by performing multiple injections. European patent EP 1 035 314 A2; 3) N. Rodrigues-Amaya, et. al. (2002) Method for injection fuel with multiple triggering of a control valve. Robert Bosch GmbH, U.S. patent No. 2002/0083919 A1; 4) M. Brian, Caterpillar Inc. (2002). Method and apparatus for delivering multiple fuel injection to the cylinder of an engine wherein the pilot fuel injection occurs during the intake stroke. Intentional patent WO 02/06652 A2; 5) K. Yoshizawa, et. al., Nissan Motor Co., Ltd (2001). Enhanced multiple injection for auto-ignition in internal combustion engines. Us Patent US 2001/0056322 A1; 6) Y. Wang et. al., Ford Motor Company and K. S. Peterson et. al., University of Michigan (2002). Modeling and control of electromechanical valve actuator. SAE International,SP-1692, 2002-01-1106, 43-52; and 7) V. Giglio et. al. (2002). Analysis of advantages and of problems of electromechanical valve actuators. SP-1692, 2002-01-1106, 31-42.
Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying figures. The figures constitute a part of this specification and include illustrative embodiments of the present invention and illustrate various objects and features thereof.